1. Field of the Invention
The present invention relates to transmission construction. In particular, the invention relates to a transmission for driving a vehicle provided with a crawler type traveling unit.
2. Related Art
Generally, a transmission used for left and right crawler type moving devices is hydraulically driven by a pair of hydraulic motors. The transmission includes a pair of variable displacement hydraulic pumps mechanically connected to an engine on a vehicle. Fixed or variable displacement hydraulic motors, which are fluidly connected with each other to be independently driven by the pair of hydraulic pumps, are laterally disposed. Output shafts of the hydraulic motors are connected through drive trains to left and right axles connected to the left and right crawlers.
The speed of the transmission is controlled by a steering unit for changing the capacities of the hydraulic pumps. To move the vehicle straight forward, the steering unit is put in a straight forward traveling position and the hydraulic motors are controlled such that the number of rotations of each hydraulic motor is equal. As a result, the vehicle should move forward, however, for example, on a marsh land, the number of rotations of left and right crawlers tend to vary, causing the vehicle to unexpectedly meander due to an imbalance in the vehicle's center of gravity when the vehicle leans or tilts, as well as, due to imperfections in the design and construction of the vehicle.
To prevent the vehicle from meandering as discussed above, U.S. Pat. Nos. 2,996,135, 3,655,004 and 3,744,584 provide a clutch to restrict the left and right crawlers. In particular, each clutch is engaged with each other during straight forward movement and both crawlers are forced to rotate at equal rotational speeds to improve the accuracy of the forward movement.
Each clutch engages when the steering unit is in the straight forward travel position for straight forward movement of the vehicle and disengages when the steering unit is in the steering position, that is when the vehicle is being steered in a direction other than straight forward. For example, when the steering unit is immediately restored to the straight forward travel position from the steering position, the number of rotations of the left and right hydraulic motors are temporarily non-coincident with each other due to the play of the link which couples the steering unit with an operating unit of the hydraulic pump. When the clutch engages in this situation, the relative rotation of the motors may subject the vehicle to impact or the clutch may be damaged. Hence, the clutch must be strong enough to withstand such impact and have a sufficiently large capacity. Accordingly, to accommodate the higher reduction ratio, a larger amount of clutching is necessary to transmit a high torque. Therefore, the clutch has to be large-sized.
Also, although not discussed in the above-described patents, a parking braking unit is provided to prevent the vehicle from moving when stopped. However, for the type of vehicle having a pair of hydraulic motors connected through drive trains to left and right crawlers, the left and right crawlers must be braked essentially individually. Therefore, a parking braking unit must be provided for each drive train, thereby increasing the number of parts, which increases expense. Furthermore, to accommodate the higher reduction ratio of the drive train, a larger braking capacity is required.